This invention relates to methods for making semiconductor devices, and more particularly, to methods for making physically small microwave devices such as IMPATT diodes and BARITT diodes.
The IMPATT diode, a device primarily used for generating microwaves, is described, for example, in the U.S. patent of B. C. De Loach, Jr. and R. L. Johnston U.S. Pat. No. 3,270,293, assigned to Bell Telephone Laboratories, Incorporated and the paper "The IMPATT -- a Solid-State Microwave Generator," Bell Laboratories Record. K. D. Smith, Volume 45, 1967, page 144. An analogous device operating on the same general principle is described in the U.S. Pat. No. to Read 2,899,652, assigned to Bell Telephone Laboratories, Incorporated. A device that operates in a manner comparable to the IMPATT diode but which has better noise characteristics and is better adapted to microwave amplification is described in the U.S. Pat. No. of D. J. Coleman and S. M. Sze 3,673,514, assigned to Bell Telephone Laboratories, Incorporated, and is now commonly known as the "BARITT" diode, an acronym for Barrier Injection and Transit Time.
Both BARITT and IMPATT devices typically comprise a small silicon chip having a gold "front contact" on one side and a gold "back contact" on the other side. Good electrical characteristics and adherence are typically obtained by including a silicon-containing metal layer such as platinum-silicide between each of the gold contacts and the silicon chip.
These devices are typically very small, and as the intended frequency of operation increases, the physical device size decreases and fabrication tolerances become more stringent. Thus, sophisticated photolithographic masking and etching techniques are used to make a plurality of devices from a single silicon wafer coated with gold on opposite sides. A gold etchant may be used to cut through the gold front contact, a silicon etchant to cut through the wafer, and a second gold etchant to cut through the back contact. Several problems, however, have been encountered. First, it is difficult to remove by etching the silicon-containing metal layer (such as platinum-silicide) without seriously undercutting the gold front contact. Alternative methods for removing the platinum-silicide such as liquid honing produce other undesirable effects. For example, in liquid honing, a stream of abrasive liquid is directed at the front contact to erode away the platinum-silicide and the silicon, using the gold front contact as a mask. This technique, however, tends undesirably to bend and distort the front contacts.
After the silicon wafer has been etched, it is difficult to etch the gold back contact from the side opposite the silicon wafer since this requires precisely registered photolithographic masking, and with such small devices, accurate registration is difficult.
Other conventional techniques for separating the devices from a common wafer such as "dicing" by sawing or other cutting techniques are not sufficiently delicate or sophsiticated to form the extremely small devices desired with the required accuracy. The microwave frequency IMPATT and BARITT diodes contemplated by the present invention typically have diameters of less than 10 mils and thicknesses of typically less than 40 micrometers. BARITT diodes may have a diameter of less than 2 mils with a thickness of only 15 micrometers.